Global warming will displace millions of people, trigger falling crop yields, stoke conflict and cost trillions of dollars in lost economic output, a United Nations report will warn.

A draft of the report to be finalised later this month by the Intergovernmental Panel on Climate Change (IPCC) and obtained by The Independent in Britain, says “hundreds of millions of people” will be forced to move because of coastal flooding and land loss as sea levels rise.

Food security will increasingly be threatened, with median crop yields to drop by as much as 2 per cent per decade for the rest of the century. Demand, though, is on course to rise 14 per cent per decade until 2050, the Independent cited the IPCC draft as saying.

Poverty and economic shocks from climate change will have a significant impact on migration, increasing the risks of violence from protests and from civil or international conflicts, according the draft version of the report on Impacts, Adaptation and Vulnerability states.

Among the projections likely to attract scrutiny from reviewers when the final report of the IPCC's Working Group II is settled on in Japan later this month is the draft's estimate that annual global gross domestic product will drop by 0.2-2 per cent if temperatures rise 2.5 degrees above pre-industrial levels.

Global mean temperatures have already risen about 0.9 degrees, with at least a 2-degree rise increasingly likely as carbon dioxide emissions from the burning of fossil fuels and land clearing continue to rise, scientists say.

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Based on 2012 figures of GDP at $US71.8 trillion ($A79 trillion), the annual loss would be as much as $1.4 trillion (A$1.54 trillion), the Independent said.

The aftermath of super typhoon Haiyan last November in Leyte, Philippines. Photo: Getty

A report released this week by several US groups, including the Environmental Defence Fund, though, argued economic impacts from climate change are more difficult to predict than changes in nature itself. Models deal better with rising sea-level and temperature predictions, for instance, than the impacts of extreme weather events such as droughts and floods, according to the report, Omitted Damages: What's missing from the social cost of carbon.

Biodiversity loss is also hard to value, the report said. Famine, political unrest and migration, which are often the result of multiple stresses “are usually omitted because they are difficult to quantify, predict and value”, the study found.

Food issues

Food security is among the key issues involving climate change impacts. A report published this week in Nature Climate Changeby scientists including Mark Howden from the CSIRO found yield losses for wheat, rice and corn output are expected for 2 degrees of local warming if farmers didn't act to reduce damage.

“(T)he impact of climate change on crops will vary both from year-to-year and from place-to-place – with the variability becoming greater as the weather becomes increasingly erratic,” said the report's lead author, Leeds University Professor Andy Challinor, according the university's website.

Impacts will be increasingly negative on crop yields from the 2030s onwards, with decreases of more than 25 per cent increasingly common, the website said, adding that the impacts were likely to be felt earlier than had previously been expected.

Macquarie University's Professor Lesley Hughes, a lead author of the IPCC's up-coming report, declined to comment on its contents but said scientists have increasing confidence the climate is shifting both from research and a rise in observations of extreme events.

“The climate system is very different from what it was three or four decades ago,” Professor Hughes said, citing the incidence of heatwaves and droughts among the changes.

The summary of the draft report alone runs for 76 pages, with the full 30 chapters extending for hundreds more. The draft summary notes that the number of papers on adaptation to climate change had doubled in the five years to 2010, adding to the material to be assessed by the reports' authors.